The Collaborative International Dictionary of English v.0.48:

Seigniory \Seign"ior*y\, n.; pl. -ies. [OE. seignorie, OF.
   seigneurie, F. seigneurie; cf. It. signoria.]
   1. The power or authority of a lord; dominion.
      [1913 Webster]

            O'Neal never had any seigniory over that country but
            what by encroachment he got upon the English.
                                                  --Spenser.
      [1913 Webster]

   2. The territory over which a lord holds jurisdiction; a
      manor. [Written also seigneury, and seignory.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

seigniory
    n 1: the estate of a seigneur [syn: seigneury, seigniory,
         signory]
    2: the position and authority of a feudal lord [syn:
       seigniory, seigneury, feudal lordship]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

SEIGNIORY, Eng. law. The rights of a lord as such, in lands. Swinb. 174.